Sometimes the viscosity of a fluid will change with agitation. Some fluids thicken with agitation while other fluids thin out with agitation.

Newtonian fluids are those fluids that do not change their viscosity regardless of the agitation.

The name was coined in honor of Sir Isaac Newton. You will remember he was the one that had the apple fall on his head.

Newtonian fluids flow immediately on the application of a force and the rate of flow is directly proportional to the force being applied.

Water and gasoline are examples of Newtonian fluids.